First stab

server/remark-includes.ts

@@ -155,8 +155,6 @@ export function parsePartialParams(expr: string): ParameterAssignments {
 // The default assignments expression must be on the first line of a partial.
 // Values must be wrapped in double quotes and separated by single spaces.
 export function parseParamDefaults(expr: string): ParameterAssignments {
-  const defaultAssignmentRegexp = new RegExp("{{ (.*) }}", "g");
-
   // Callers should handle empty values before they get here.
   // parseParamDefaults has no idea whether this case is acceptable or not.
   if (expr === "") {
@@ -167,13 +165,28 @@ export function parseParamDefaults(expr: string): ParameterAssignments {
     return {};
   }
 
+  const possibleAssignmentRegexp = new RegExp("{{ (.*) }}", "g");
+  const paramUseRegexp = new RegExp("{{ (w+) }}");
   const firstLine = expr.split("\n", 1)[0];
-  const matches = defaultAssignmentRegexp.exec(firstLine);
+  const firstLineMatches = possibleAssignmentRegexp.exec(firstLine);
+  const paramUses = paramUseRegexp.exec(expr);
 
-  if (!matches) {
-    return {};
+  let assignments: ParameterAssignments = {};
+  // Treat this as an attempted default parameter assignment
+  if (!!firstLineMatches && firstLineMatches[1].includes("=")) {
+    assignments = parseAssignments(firstLineMatches[1]);
+  }
+  if (!paramUses) {
+    return assignments;
   }
-  return parseAssignments(matches[1]);
+  // The default value for parameters without a default is the empty string.
+  paramUses.slice(1).forEach((use) => {
+    if (!assignments[use]) {
+      assignments[use] = "";
+    }
+  });
+
+  return assignments;
 }
 
 // resolveParamValue takes the quoted value of a template variable within a